function [ ] = mx_eyelink_playback_start( )
%MX_EYELINK_PLAYBACK_START Flushes data from queue and starts data playback. An
% EDF file must be open and have at least one recorded trial. Use 
% eyelink_wait_for_data() to wait for data: this will time out if the playback
% failed. Playback begins from start of file or from just after the end of the 
% next-but-last recording block. Link data is determined by file contents, not 
% by link sample and event settings.
%
% Returns:
% 0 if command sent OK, else link error.
%
%----------------------------------------------------------------------------
% Copyright Eduardo Aponte 2013
% aponteeduardo@gmail.com